Technical Field
Embodiments of the subject matter disclosed herein generally relate to methods and systems and, more particularly, to mechanisms and techniques for improved azimuth distribution in seismic data acquisition.
Discussion of the Background
Marine seismic data acquisition and processing generate a profile (image) of a geophysical structure (subsurface) under the seafloor. This profile does not necessarily provide an accurate location for oil and gas reservoirs, but it may suggest, to those trained in the field, the presence or absence of oil and/or gas reservoirs. Thus, providing a high-resolution image of the subsurface is an ongoing process.
For a seismic gathering process, as shown in FIG. 1, a data acquisition system 10 includes a vessel 12 towing plural streamers 14 that may extend over kilometers behind the vessel. One or more source arrays 16 may be also towed by the vessel 10 or another vessel for generating seismic waves. Conventionally, the source arrays 16 are placed in front of the streamers 14, considering a traveling direction of the vessel 10. The seismic waves generated by the source arrays propagate downward and penetrate the seafloor, eventually being reflected by a reflecting structure (not shown) back to the surface. The reflected seismic waves propagate upwardly and are detected by detectors provided on the streamers 14. However, such a method results in data having poor azimuth distribution.
An improvement to this conventional data acquisition method is the use of wide-azimuth (WAZ) acquisition. In a typical WAZ survey, two streamer vessels and multiple sources are used to cover a large sea area, and all sources and streamers are controlled at a uniform depth throughout the survey. WAZ acquisition provides better illumination of the substructure and, thus, a better final image. However, the presence of ghosts (e.g., reflections of waves from the surface of the water back to the receivers of the streamers) in the acquired data still affects the final image due to the presence of notches.
A newer approach, rich-azimuth (RAZ) acquisition, shows promising signs for improving the final image. RAZ acquisition is the combination of multi-azimuth acquisition and wide-azimuth geometry. RAZ acquisition may be implemented by shooting a same cell in three directions, e.g., 30°, 90°, and 150°, each direction being shot in one or two passes. A rose diagram for such a rich-azimuth survey shows the benefits of rich-azimuth towed-streamer acquisition, i.e., azimuth coverage from 0° to 360° and uniform offset distribution from 400 m to 8000 m for a 8000 m long streamer.
However, the existing RAZ acquisition can further be improved to increase the illumination and accuracy of the final image by finding an appropriate number and distribution of source arrays to achieve ultra long offset together with broadband techniques. Accordingly, it would be desirable to provide systems and methods that avoid the afore-described problems and drawbacks, and improve the accuracy of the final image.